I had a few customers bringing their Toshiba laptops for repair with exactly the same problem. The touchpad stopped working without any reason. The laptop is working just fine with an external USB mouse but the touchpad is dead.
Solution? Most likely the touchpad was disabled somehow. On Toshiba laptops you can enable or disable the touchpad with Fn+F9 shortcut on the keyboard.
Hold down the Fn key and at the same time press once on the F9 key. This will enable the touch pad.
Does it help?